Possible Role of Volatile Fatty Acids and Abscisic Acid in the Dormancy of Oats

Abstract

Species of Avena differ markedly in their levels of pre-and post-harvest dormancy. These species offer the opportunity of determining if dormancy is related to the endogenous level of growth inhibitor. Germinability in two species of differing levels of dormancy, common oat Avena sadva L., and wild oat Avena fatus L. was assessed as were the contents of abscisic acid and volatile fatty acids of chain length C6-ClO. In A. sativa which did not possess postharvest dormancy there was no correlation between germination and inhibitor levels but in A. fatua the relationship between the content of fatty acid and dormancy was good. The loss of these fatty acids in dry storage by evaporation could explain after ripening.In oats (A vena spp.) there is a considerable range of degrees of dormancy from the almost nondormant common oat (A. sativa L.) which at most shows a brief postharvest dormancy, through the appreciable dormancy of the wild oat (A.fatua L.) to the profound dormancy of the winter wild oat (A. ludoviciana Dur.), especially with respect to the upper grain of the dissemule. The level of dormancy in this last species during maturation is less than when fully ripe (17,21). However, Morgan and Berrie (17) showed that if immature grains were dried before testing these dried grains showed a degree of dormancy equivalent to the mature grain.Wright (25) showed that water-stressed plants had elevated levels of an ABA-containing complex, possibly produced to control stomatal aperture (12). It might be concluded that ABA would accumulate in plant tissues in which the water contents were reduced compared to average normal tissues. Inasmuch as ABA is a well known germination inhibitor, such accumulation could explain the pattern of emergence found in maturing A vena spp.With the possible exception of ABA, naturally occurring plant growth inhibitors are considered to be nonspecific (15). This includes the coumarins which can impose light sensitivity on lettuce seed (4). Fatty acids of chain length C6-C12 occur in many plants and influence seed germination and hydrolase production in a manner contrary to known promotive plant hormones (3,9 Ripe grains were stored at a constant temperature of 20 C in darkness until required for analysis.Single large batches of grain were collected. Prior to beginning the routine extractions, a model system employing replication at all levels was carried out for each acid. Isotope dilution was employed to estimate efficiency of extraction and recoveries were also estimated by "seeding" samples with known amounts of acid. From these experiments the coefficient of variation for each acid was determined. In the working analyses the final analysis was carried out in triplicate and the average of these was employed, using the appropriate coefficient of variation to estimate the standard deviation. It is this value which is reported in the tables.Extraction

Role of Endogenous Growth Regulators in Seed Dormancy of Avena fatua

“…No decanoic (C 10) acid was detected in either dormant or nondormant seeds. This is in sharp contrast to the observation by Berrie et al (2) who found that decanoic acid formed the major portion of the total short chain fatty acid fraction (C 6-C 10).…”

“…We did not observe the correlation between endogenous levels of short chain fatty acids (C 6-C 10) in seeds or embryonic axes and dormancy as was reported by Berrie et al (2). While these discrepancies are difficult to explain, it is important to note that the seed stocks used in both studies were genetically heterogeneous, and from different areas with divergent environments.…”

“…Recently, it has been proposed that volatile fatty acids of chain length C 6-C 10 play a major role in the induction and maintenance of seed dormancy in wild oat by acting as natural germination inhibitors (2). The reasoning behind this hypothesis was 3-fold: first, fatty acids, particularly C 7, C 8, and C 9, were moderately effective inhibitors of seed germination in A. fatua (3).…”

“…Third, and most importantly, there was a strong correlation between the levels of endogenous short chain fatty acids and the degree of dormancy in seeds of wild oat (2). These workers suggested that loss of the fatty acids by evaporation from the seeds could explain the basic mechanism underlying the afterripening process (2).…”

Studies in Wild Oat Seed Dormancy

“…Recent interest has also been centered around the role of volatile fatty acids (3). At present, very little is known concerning the mechanism by which endogenous C2H4 acts in dormancy breakage and early germination of this species.…”